Variable resistor



OC 24, 1944. H. H. CARY ETAL VARIABLE RES ISTOR iled June 25, 1941 2 Sheets-Sheet l Oct. 24, 1944. H. H. CARY ET AL VARIABLE RESISTOR 2 Sheets-Sheet 2 Filed June 23, 1941 WMMW WIW

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5 /NVE/VTORS HENRY H. CARY Doaamqs GeoRGE MAR/ Ow Patented Oct. 24, 1944 UNITED'STATES PATENT OFFICE VARIABLE RESISTOR Application June 23, 1941, Serial No. 399,300

(C1. 2in-5c) 2i) Claims.

Our invention relates to resistances for electric circuits, with special reference to variable resistors for service as rheostats, potentiometers, voltage dividers, and the like.

The general object of our invention is to provide an exceptionally compact variable resistor offering finely graduated control over a relatively extensive range of resistance values. A further general object is to provide such a variable resistor that is structurally efficient, dependable in service, and relatively inexpensive to manufacture.

The invention includes a resistive conductor wound into a spiral, and in the preferred iorm of the invention the resistive conductor is wound into a double spiral comprising a minor spiral of relatively numerous small-diameter turns and a major spiral of relatively few large-diameter turns formed by the minor spirali In some practices of our invention the major spiral may have the conguration of a plane curve formed by a point that moves around a fixed center and continually increases its distance from the center; in other practices of our invention the spiral may define a cylindrical configuration like a screwthread. Various objects of our invention relate 'to the manner of arranging a movable contact for cooperation with a spiral resistance- Among these objects are: to employ the spiral conductor as a mechanical guide for the Contact; to limit the area of electrical contact with the spiral by providing a separate non-conductive guide element for mechanical engagement with the spiral; to reduce friction and wear by providing for rolling engagement of the non-conductive guide element with the spiral; and to provide a separate guide element without limiting the range of spiral movement lof the contact along the spiral.

A further object in mind is to provide for substantially constant mechanical pressure against the spiral resistance over the entire range of contact movement. In this regard our invention is characterized by the concept of movM ably mounting the contact on a support that. in eiiect, iicats in a position transversely of and eouidistant from the turns of the spiral resistance. More specific objectsy to appear later relate to the manner of yieldingly mounting such a contact-support to provide the desired constancy of pressure on the part of the movable Contact.

Further objects and advantages of our invention will be apparent from the detailed description to follow taken with the accompanying drawings.

In the drawings which are to be considered as illustrative only:

Fig. 1 is a longitudinal section through a preferred embodiment oi our invention;

Fig. 2 is an end view of the device taken as indicated by the arrow 2 in Fig. l, a casing cover being removed to reveal the interior of the devlce;

Fig. 3 is a transverse section taken as indi cated, by the line 3-3 of Fig. 1;

Fig. 4 is a transverse section taken on the broken line 4 -4 of Fig. l;

Fig. 5 is a longitudinal section taken as indicated by the line 5-5 of Fig. l;

Fig. 6 is a phantom view showing Selected interna] elements in perspective;

Fig. 7 is a sectional View of a second form of our invention;l

Fig. 8 is a section taken as indicated by the broken line 8 8 of Fig. 7;

Fig. 9 is a sectional View of a third invention; and

Fig. l0 lis a section taken as indicated by the line lil-l0 of Fig. 9.

The principal parts of the preferred form of our invention shown in Figs. 1 to 6 include a spiral resistance generally designated 20 in a cylindrical casing 2|, the two comprising a resistance assembly, a rotary control means generally designated 22 including an operating knob form of our 23 and an operating shaft 24, a contact-supportv generally designated 25 carried by the rotary control means, and a contact assembly generally designated 21.

The cylindrical casing 2l is closed at one end by a cover 28 which is normally retained in a releasable manner by a resilient split ring 30, the cover and the cylindrical casing being provided with complementary grooves 3l and 32 respectively to seat the ring. The cylindrical casing 2| is also provided with a tubular member 33 that serves as a bearing for the operating shaft 2Q and is externally threaded to receive a nut 35. Such a construction permits the device to be mounted on a panel 36 in the manner indicated in Figs. 1 and 5, the tubular member 33 extending through a panel aperture 31 and the panel being gripped between the nut 35 and an annular shoulder 38 of the tubular member. The operating knob 23 is secured on the operating shaft 24 by a suitable setscrew 40 after the cylindrical casing 2| is mounted on the panel.

The spiral resistance 20 preferably is a relawire may be said to have the form of a double spiral in which the turns 42 of relatively small diameter on the core form a minor spiral and the turns 45 of the core of larger diameter form a major spiral. The core 43 may be fabricated from any suitable material, but in the construction shown comprises a relatively heavy copper wire with an adequate coating of -insulating material. The relatively fine wire forming the smalldiameter turns 45 on the core 43 is a resistive conductor with an insulating coat that is partially removed after the winding operation to expose the wire for cooperation with the contact assembly 21. It will be noted that both the minor spiral and the major spiral are closely wound to make an exceptionally compact device and that an exceedingly large number of the small turns 42 provide exceptionally close control over an exceptionally extensive range of resistance values.

One or both ends of the resistance may be connected to external terminals on the cylindrical casing 2|. such terminals 45 are provided, the terminals being in the form of metal clips extending through the wall of the cylindrical casing. While the spiral resistance 2Q may be mounted in the cylindrical casing 2| in any suitable manner in the various practices of our invention, one feature of our preferred practice is the conception of securing the spiral resistance in the casing by expanding the spiral into engagement with the casing. In the illustrative construction. one end of the core 43 rests against a stop shoulder 41 formed in the cylindrical casing 2| and the other end of the spirally wound core abuts a second stop shoulder 48 provided by a retaining ring or sleeve 50 that is telescoped into the cylindrical casing. In the assembly procedure, the spiral resistor is placed loosely in the cylindrical casing 2| with the two terminals 4B extending through suitable slots 5| in the casing wall and then the retaining sleeve 5D is inserted and rotated against the outer end of the spiral core 43 to expand the spiral. The slots 5l are cut to permit some latitude in the location of the two terminals 4.6 and to avoid interfering with the transmission of pressure through the spiral resistance in this assembly procedure. After the retaining sleeve 50 is rotated against the end of the resistor to create the desired expanding pressure, the retaining sleeve is anchored in place by suitable means such as one or more pins 52 penetrating the cylindrical casing 2|.

In the present form of our invention the con.- tact-sunport includes a polished rod 53 on which the contact assembly 21 is sldably mounted. It is contemplated that the polished rod 53 will be mounted on the rotary control means 22 in a yielding or floating manner, but will always be maintained substantially parallel to the axis of the maior spiral, i. e., the axis of the spiral formed by the core 43. The purpose of yieldingly mountinsr the polished rod 53 and at the same time holding the rod in the parallel relationship is to provide for uniform pressure of the Contact assembly against the spiral resistance throughout the entire range of adjustment. While various mechanical arrangements may be employed for mounting the polished rod 53 on the rotary control means 22 in the required manner we prefer to employ a pair of resiliently biased In the present construction two f parallel arms 55, and while the two arms may be biased by resorting to Various expedients we prefer to make the two arms of resilient material and thereby avoid the necessity of providing separate resilient means.

In the construction shown, the inner end of the operating shaft 24 carries what may be termed a rotary base member 56 of non-conductive material on which the two arms 55 are mounted by a suitable screw 51 and small plate 58, the arms comprising flexible metal strips with bent ends E0 overlapping between the base member and the small plate. The outer ends of the flexible arms 55 carry a head member 6| which in turn carries the previously mentioned polished rod 53 by one end. The head member 5| comprises a metal bar and is attached to the two arms 55 by a rivet 62, the rivet penetrating bent ends 63 of the two arms and a small retaining plate E5. Since the two arms 55 are resilient, they permit the polished rod 53 to move relative to the surroundinfT spiral resistance 20 and the fact that the two arms are parallel and also of equal length causes the polished rod to be maintained substantially equidistant from al1 of the large-diameter turns 45 of the spiral resistance. Preferably the rotary base member 55 provides a slot 66 into which the free end of the polished rod 53 extends, the slot permitting the end of the polished rod to move freely toward and away from the spiral resistance 23 b-ut preventing any substantial lateral movement with respect to the rotary control.

In designing the Contact assembly 21, we contemplate arranging an electrical contact to move freely along the length of the polished rod 53 and to be guided in such movement .by mechanical engagement with the large-diameter turns 45 of the spiral resistance. The electrical contact and the means for guiding the electrical contact by mechanical engagement with the spiral resistance may be the same member, i. e., an electrical contact may be designed for guiding engagement with the spiral resistance; it is desirable, however, to provide a separate guide member of nonconductive material operatively connected with the electrical contact. It may be noted that the spiral resistance 20 is closely wound both with respect to the small-diameter turns 42 and the large-diameter turns 45 for the sake of compactness. It is difficult to combine in one member the function of contact and contact guide means for cooperation with such a closely wound spiral resistance without causing electrical contact to be made simultaneously with several of the small-diameter turns 42 in two or more of the large-diameter turns 45, a condition that is undesirable in some electrical installations. We prefer, then, to employ a separate non-conductive guide means and further prefer to design such guide means for rolling contact with the spiral resistance 20.

In the particular construction shown by way of example in Figs. 1 to 6, a contact-carrier 10 has two spaced legs 1| with an interconnecting sleeve 1'; for sliding engagement with the polished rod 53 and has an arm 13 slidingly extending into a slot 14 in one of the parallel arms 55 (Figs. 3 and G) to prevent rotation of the contact-carrier about the axis of the polished rod. Rotatably and slidingly mounted on the polished rod 53 between the two legs 1| of the carrier is a guide member in the form of a wheel 15 of nonconductive material, which wheel is preferably tapered at its periphery to a thin edge to seat into the spiral groove that is formed by the large-diameter turns 45 of the spiral resistance 20. The spiral resistance serves as a track for the wheel. A contact in the form of a resilient metal arm 'i6 with an end curved' as shown is mounted across the ends of the two carrier legs 1| to follow the spiral path defined Iby the largediameter turns 45 and thereby shift from one of the small-diameter turns 42 to another over the length of the resistance Current may be taken from the contact l@ by any suitable means, not shown in detail, electrically connected to any metallic portion of the rotating structure, such means being well known in the art. For example, a brush contact may engage the head member 6| in this connection.

Preferably the retaining sleeve 50 is formed with an inner spiral groove I8 in continuation of the spiral groove formed by the spiral resistance 20, so that the guide wheel 'l5 may overrun the end of the spiral resistance and thereby permit the Contact l to reach the eXtreme end of the spiral resistance. We further prefer to provide suitable stop means for abutment by the contact-carrier l when trie contact l5 reaches either end of the spiral resistance. show in the drawings a stop shoulder 80 formed in the cylindrical casing 2l and a second stop shoulder 8| provided by the retaining sleeve 50.

The operation of this first form of our invention may be readily understood from the foregoing detailed description, It is apparent that rotation of the knob 23 causes the contact 'f6 to move along a spiral path conforming to the spiral configuration of' the resistance and that relatively close adjustment of resistance values may be had over a relatively extensive range of values. Since the pressure of the Contact assembly toward the surrounding spiral resistance is provided by the resilient parallel arms 55 and since the polished rod 53 is substantially equidistant from the several large-diameter turns 45 of` the resistance, the pressure exerted through the contact carrier "lil is substantially uniform throughout the whole range of adjustment.

In the second embodiment of our invention depicted in Figs. 7 and 8, We apply the same principles to an arrangement in which a contact moves around the exterior of a spiral resistance. Fig. 7 shows a cylindrical casing mounted on a panel 86 in the previously described manner, an operating shaft El extending through the panel and carrying the usual operating knob 85.5. The cylindrical casing lili, which is provided with a removable cup-shaped cover 353, an interior cylindrical wall 9i that is embraced by a spiral resistance 92 and by sleeve 33, The resistance 92 which is connected to suitable external terminals (not shown) is secured on the cylindrical wall 9| in any suitable manner and is similar in construction and arrange"r- .nt to the previously described spiral resistance the coniguration including a similar minor spiral of small-diam eter turns and a major spiral of large-diameter turns.

The means for supporting a movable contact in the arrangement shown in Figs. 'l' and 8 includes a base member 35 xedly embracing the operating shaft 8l, a pair of flexible arms 96 mounted in parallel relation on the base member, and a head member Sl mounted on the ends of the flexible arms ltl. The head member Sl has a relatively large aperture 98 surrounding the operating shaft 3l to permit movement relative to the operating shaft and carries at its outer end a polished rod |00 that is positioned outside the spiral resistance 92 and disposed parallel to the operating shaft 8l. A contact-carrier |0| that is slidingly mounted on the polished rod |00 carries both the usual contact |02 and guide wheel |02 and is provided with a short arm |05 to prevent rotation about the axis of the polished rod. In this construction the short arm |05 extends into a guide slot |06 in a special guide bar |07 that is non-rotatively retained on the end of the operating shaft 81 by a suitable nut |08. Preferably the guide bar |01 is provided with an inwardly bent end ||0 that has a guide slot for the free end of the polished rod |00. The two flexible arms 96 are biased to urge the polished rod |00 toward the cylindrical wall 9| for the purpose of maintaining the guide wheel |03 in the groove formed by the spiral resistance and for the further purpose of rholding the contact |02 against the spiral resistance.

The third form of our invention shown in Figs. 9 and l0 is based on the novel concept of arranging a double-spiral resistance in substantially one -plane for installations where available space for the variable resistor is limited in one dimension, A casing ||2 including a removable 'cup-shaped cover ||3 is shown mounted in the previously described manner on a panel H5. A spiral resistance |.I6 comprises a coated wire Wound on a core 1 that has the configuration of a flat spiral rather than a ycylindrical spiral. As best shown in Fig. 10 this arrangement provides numerous small-diameter turns ||8 of a minor spiral and larger turns of a major spiral The spiral resistance i6 may be secured in place in any suitable manner and attached to suitable external terminals (not shown). In the particular arrangement shown, the spiral resistance i |55 is surrounded by a ring |2| of non-conducting material having a spiral groove |22 in continuation of the spiral groove formed by the re sistance.

An operating shaft |23 carrying the usual operating knob |25 is journaled in one bearing provided by a ,tubular member |20 and in a sec ond bearing B27 in the casing cover H3, Suitably mounted on the operating shaft |23 in a manner permitting movement toward away from the spiral resistance ||6 is a contact-support arm |30. As shown in Fig. 9 the arm |30 may be carried by a ring I3! that is slidingly keyed to the operating shaft |23 and is integral with an over-sized cylinder |32 surrounding the shaft. One bearing point for supporting the arm is provided by the ring |36, and a second bearing point is provided by a collar |33 that is threaded onto the operating shaft |23 inside the cylinder |32. A suitable spring |35 embracing the operating shaft |23 between the ring 13| and the collar |33 serves as the required means for urging the contact-support arm |31@ toward the plane of the spiral resistance i6,

The contact'support arm |30 engages both ends of a polished rod i3d and is formed with a portion i3'.I parallel with the polished rod. usual contact-carrier |40 is slidingly mounted on the polished rod |36 and prevented from rotation by an arm Ml that extends into a guide slot M2 in the portion |37 of the support The contact-carrier |40 has the usual contact |43 pressing against the spiral resistance i it and has the usual guide wheel Hill in mechanical gagernent with the spiral resistance. Rotation of the operating shaft |23 by manipulation of the operating knob causes the contact M3 to sweep in a at spiral path conforming to the configuration of the fiat spiral resistance, and uniform pressure against the spiral resistance throughout the range of adjustment is maintained by the concealed spring |35.

The three selected embodiments of our invention described in detail herein for the purpose of disclosure Will suggest to those skilled in the art various changes and substitutions under our inventive concept; We reserve the right to all such departures from our described arrangements that come Within the scope of our appended claims.

We claim as our invention:

1. A variable resistor having: a spiral resistive conductor, adjacent turns of the spiral forming a spiral groove; a contact movably touching a turn of said spiral conductor; a guide member of nonconductive material movably extending into said spiral groove, said guide member being connected with said contact to coniine the contact to move in a spiral path conforming to the spiral turns of the conductor and in such manner that said contact engages one turn of said spiral conductor at a time vvhil3 advancing along said spiral pathl guided by said guide member; and means for advancing said guide member progressively along said spiral groove.

2. A variable resistor having: a resistive conductor Wound to form a spiral; control means rotatable about the axis of said spiral; a contactsupport extending across the turns of said spiral in substantially equidistant spacing from the individual turns; means including a parallel linkage mounting said contact-support on said control means to control the movement of the contact-support relative to said spiral While maintaining said equidistant spacing from the turns of the spiral, said means including means yieldingly urging said contact-support toward said spiral; and a contact movably mounted on said contact-support and mechanically guided by the turns of said spiral to move along and in engagement with said spirally-Wound conductor.

3. A variable resistor having: a stationary resistive conductor Wound to form a spiral; control means rotatable about the axis of said spiral; a contact-support traversing the turns of said spiral in spaced relationship therewith and rotatable with said control means; means for mounting said contact-support to move in a direction toward and away from said spiral; yielding means for urging said contact-support toward said spiral; a contact movably mounted on said contact-support and engaging a turn of the spirallywound conductor; a guide member movable along and in contact with the turns of said spiral; and means for operatively connecting said guide member and said contact to move said contact along said spirally-wound conductor as said guide member advances along the turns of said spiral.

4. A variable resistor having: a resistive conductor Wound to form a spiral having an end; a

contact movably touching one turn of said spiral member beyond the end of the spiral to permit said contact to reach the end of the spiral.

5. A variable resistor having: a resistive conductor Wound to a spiral forming a spiral groove; a contact movably touching a turn of said spiral; a Wheel riding in said groove, said wheel being connected with said contact to guide the contact; and control means to cause relative rotation of said contact and Wheel with respect to said spiral about the axis of said spiral.

6. A variable resistor having: a resistive conductor wound to a spiral of closely adjacent turns forming a narrow spiral groove; a contact movably touching a turn of said spiral; a Wheel of non-conductive material riding in said groove for guidance by two adjacent turns of the spiral, said Wheel being connected with said contact to guide the contact; and control means to move said contact and Wheel ytogether about the axis of said spiral.

7. A variable resistor having: a conductor Wound substantially equidistant to an axis to form a spiral about said axis; rotary control means coaxial with said spiral; a pair of parallel arms on said control means; a contact-support supported by said arms in parallel relation with the axis of the spiral; and a contact assembly slidingly mounted on said contact-support including means engaging the turns of said spiral to advance said contact assembly along said contact-support, said contact assembly including a contact engaging a turn of the spirally-wound conductor; and means for resiliently biasing said contact-support toward the spiral.

8. A variable resistor having: a conductor Wound substantially equidistant to an axis to form a spiral about said axis; rotary control means coaxial with said spiral; a pair of parallel arms on said control means; a contact-support supported by said arms in parallel relation with the axis of the spiral, said parallel arms being resiliently biased to urge said contact-support toward the spiral; a contact slidingly mounted on said contact-support for electrical engagement with a. turn of the spirally-wound conductor; and a guide member formed of non-conducting material engaging said spiral to be mechanically guided by the turns of said spiral, said guide member being operatively connected to said contact to guide same along said spirally-Wound conductor.

9. A Variable resistor having: a resistance assembly including a conductor forming a spiral; control means rotatable about the axis of said spiral; a contact-support extending across the turns of said spiral in substantially equidistant spacing from the individual turns of the spiral, said contact-support being carried by said control means for rotation about the axis of the spiral; contact means mounted on said contactsupport for movement longitudinally thereof, said contact means being in contact with said conductor; and guide means spaced a. substantially constant distance from said contact means, said guide means being connected with said contact means and engaging a track provided by the spiral configuration of said conductor, there being insulating material to prevent iiow of current from the conductor through said guide means to said contact means.

10. A variable resistor having: a resistance assembly including a conductor forming a spiral; control means rotatable about the axis of said spiral; a contact-support extending across the turns of said spiral in substantially equidistant spacing from the individual turns, said support being carried by said control means for rotation about the axis of the spiral; means yieldingly `rging said contact-support toward said conductor; contact means carried by said contactsupport and extending into contact with said spiral conductor oi said resistance assembly; and contact-advancing means carried by said contactsupport to receive force exerted by said yielding means and limit the movement of the contactsupport toward said resistance assembly, said contact-advancing means being movable along said contact-support and being operatively connected with said contact means and in guiding engagement with said resistance assembly to control the movements of said contact means in response to rotation of said control means.

il. A variable resistor as set forth in claim in which said contact-advancing means is a wheel engaging said spiral conductor, there .being insulating material to prevent iiow of current from said conductor through said wheel to said contact means.

l2. A variable resistor having: a cylinder providing an inner surface; a resistive conductor wound into a spiral lying against the inner suriace oi said cylinder; stop means fixed relative to said cylinder in abutment with one end oi said spiral; a sleeve telescoped in said cylinder and rotatable therein during assembly oi the cylinder and spiral, said sleeve providing a stop surface acting against the other end of said spiral in suoli manner that rotation oi said sleeve relative to the cylinder will tend to unwind the spiral to expand against the inner surface of said cylinder, thereby to immobilize the spiral at substantially uniiorm diameter; means for locking said sleeve against rotation within said cylinder when said spiral has been expanded; a contact within said spiral electrically cooperating with said conductor; and means to move said contact adjustably along a spiral path conforming to said spiral.

1S. In a variable resistor, the combination of: a resistive conductor wound as a spiral providing a plurality of turns; control means rotatable about the axis oi said spiral; a rod extending across the turns of said spiral in substantially equidistant spacing from the individual turns; arm means operatively connecting said rod to said control means to rotate therewith, said arm means providing a portion extending transversely across said turns but spaced therirom, said portion providing a slot; a contact carrier slidable along said rod and providing an arm extending into said slot to prevent rotation of said contact carrier on said rod, said contact carrier including a contact engaging a turn of said spiral; and guide means carried by said contact carrier and engaging said spiral to advance said contact carrier along said rod to move said contact progressively along said spiral.

ill. In a variable resistor, the combination of: a resistive conductor wound as a spiral providing a plurality of aligned turns; control means rotatable about the axis of said spiral; a head member; a rod attached to said head member and disposed in a position spaced from and traversing said aligned turns of said spiral; means operatively connecting said head member and its attached rod to said control means to cause said head member and its rod to rotate about the axis of said spiral with said control means, said means including parallel-motion means for resiliently moving said rod toward said aligned turns of said spiral and for maintaining the axis of said rod substantially parallell to the aligned turns during movement of said rod toward and away from said turns; carrier means slidable along said rod and providing engagement means extending therefrom into engagement with said spirally-wound conductor to advance said carrier means along said rod aS said control means is rotated about said axis; and means for preventv ing rotation of said carrier means relative to said rod during advancement of said carrier means therealong.

i5. In a variable resistor, the combination of: a resistive conductor wound as a spiral providing a plurality of turns; control means rotatable about the axis of said spiral and providing a base member adjacent one end of said spiral; a head member adjacent the other end of said spiral; arm means extending between and operatively connecting said head member and said base member; a rod carried by said head member in a position spaced from and traversing said turns of said spiral; carrier means slidable alongsaid rod and providing a contact mounted on said carrier means and engaging a turn of said spiral, said carrier means including also an engagement means extending into engagement with said spiral to advance said carrier means along said rod, said engagement means comprising a guide roller formed of electrical insulating material rotatable on said rod and vengaging said spiral at a position spaced from the point of engagement of said contact with said turn, said guide roller being slidable along said rod, and said carrier means including means operatively connected to said guide roller to advance said carrier means and said contact in a spiral path conforming to the turns of said spiral and while said contact and said guide roller are spaced a substantially constant distance from each other; and means for preventing rotation of said carrier means relative to said rod during advancement of said carrier means therealong.

16. In a variable resistor, the combination oi: a resistive conductor wound as a spiral providing a plurality of turns; control means rotatable about the axis of said spiral and providing a base member adjacent one end of said spiral; a head member adjacent the other end of said spiral; arm means extending between and operatively connecting said head member and said base member, said arm means being flexible to permit movement of said head member relative to said base member; a rod carried by said head member in a position spaced from and traversing said turns of said spiral, said base member providing a slot into which said rod extends, said slot providing wall means guiding the movement of said rod with respect to said base member; carrier means slidable along said rod and providing engagement means extending into engagement with said spiral to advance said carrier means along said rod; and means for preventing rotation oi said carrier means relative to said rod during advancement of said carrier means therealong.

17. In a variable resistor, the combination of: a resistive conductor wound as a spiral providing a plurality of turns including two end turns terminating respectively in ends; a casing providing a surface to Contact said turns and a projection extending from said surface to be contacted by one of said end turns, said projection providing a stop shoulder engaging the end of said one end turn of said spiral; a ring rotatable relative to said casing and providing a surface providing a stop shoulder engaging the end of the other end turn of said spiral whereby turning said ring in said casing Will move the turns of said spiral into pressural contact With said surface of said casing in the Zone between said projection and said ring; a contact engagingv a turn of said spiral; and means for advancing said contact along said spiral.

18. A combination as denned in claim 1'?, in which said means for advancing said contact along said spiral comprises a guide means engaging said spiral at a position spaced from the point of engagement of said contact with said spiraly and in Which said ring provides a spiral groove continuing the configuration of said spiral beyond said stop shoulder' of said ring to guide said guide means beyond the adjacent end of said spiral to permit said contact to reach the extreme end of the spiral.

19. A combination as dened in claim 17, in which said surface o said projection of said casing is or spiral configuration to conform in shape to said one of said end turns, and in which said surface of said ring is of spiral conguration to conform in shape to the other of said end turns, and including means for anchoring said ring with respect to said casing when said ring has been turned to move the turns of the spiral into pressural contact with said surface of said casing.

2G. A variable resistor having: a resistance assembly including a conductor forming a spiral; control means rotatable about the axis of said spiral; a contact-support extending across the turns of said spiral in substantially equidistant spacing from the individual turns, said contactsupport being carried by said control means for rotation about the axis of said spiral; means for mounting said contact-support to move toward said turns of said spiral While maintaining the substantially equidistant spacing of said contactsupport from said individual turns of said spiral, said mounting means including yielding means urging said contact-support toward said conductor; Contact means carried by said contactsupport and extending into contact with said spiral conductor of said resistance assembly; and contact-advancing means carried by said contact-support to receive force exerted by said yielding means and limit the movement of the contact-support toward said resistance assembly, said contact-advancing means being movable along said contact-support and being operatively connected with said contact means and in guiding engagement with said resistance assembly to control the movements of said Contact means in response to rotation of said control means.

HENRY H. CARY. DOUGLAS GEORGE lMARLOW. 

